This invention is related generally to a directional comparison distance relay system used in a three-phase AC electric power transmission system, and more specifically, to such a relay system with prevention of over-tripping due to multiple faults.
FIG. 11 shows a typical parallel three-phase AC electric power transmission system with protection relay systems. A protection zone 10 of a length of parallel three-phase AC electric power transmission lines 12 is set between first and second substations 14a and 14b, respectively. The first and second substations 14a and 14b have protection relay systems 16a and 16b, circuit breakers 18a and 18b, voltage transformers 20a and 20b, and current transformers 22a and 22b, respectively. The protection relay systems 16a and 16b include transmission devices 24a and 24b, respectively.
FIG. 8 is a simplified diagram showing a trip sequence logic of a typical directional comparison distance relay system. In this figure, the numeral 300 represents a receiver section of a transmission device, which receives and outputs a permissive trip signal sent from the far end (opposite end) of the protective section of the electric power transmission line. The numeral 302 represents a Zone 1 element of a short-circuit distance relay which operates for a short-circuit fault occurring in the forward protection direction from the local end and within about 80 to 90% of the whole length of the protection zone. Trips due to the Zone 1 element does not require permissive trip signals from the far end.
The numeral 303 represents a forward short-circuit directional distance relay which operates when a short-circuit fault has occurred forward in the protection direction. Note that “short-circuit distance relay” in this specification means a combination of a Zone 1 element 302 of a short-circuit distance relay and a forward short-circuit directional distance relay 303.
The numeral 304 represents an “AND” gate logic which permits a trip when the output signal of the forward short-circuit directional distance relay 303 and a permissive trip signal 301 from the far end are received. The numeral 305 represents an “OR” gate logic which becomes TRUE when the Zone 1 element 302 of the short-circuit distance relay and the “AND” gate logic 304 become TRUE.
The numeral 306 represents a trip condition of the short-circuit distance relay. This condition 306 is added as an input to the “AND” gate logics 307 and 308, because there are some cases where the circuit breaker should preferably prohibited to be tripped/opened even when the Zone 1 element 302 of the short-circuit distance relay and the forward short-circuit directional distance relay 303 are in operation.
The numeral 309 represents a trip signal for the circuit breaker by the short-circuit distance relay. The numeral 310 represents a permissive trip signal, which is sent to the far end through a signal send port 311 of a transmission device. The trip condition 306 of the short-circuit distance relay will be described below in detail referring to FIG. 9.
The earth fault relay has a similar circuit breaker trip sequence logic. The numeral 312 represents a first step of the earth fault distance relay, the numeral 313 represents a forward earth fault directional relay, the numeral 314 represents an “AND” gate logic, the numeral 315 represents a trip condition of the earth distance relay, the numeral 316 represents an “OR” gate logic, the numerals 317 and 318 represent “AND” gate logics, the numeral 319 represents a circuit breaker trip command by the earth fault distance relay and the numeral 320 represents a permissive trip signal.
The typical directional comparison distance relay system has an earth fault distance relay and a short-circuit relay as in a typical case. The short-circuit relay may be activated even at an earth fault in a single line under some condition including the condition of reverse impedance. If the earth fault in a single line is tripped by the short-circuit relay, a three-phase trip is caused. Therefore, single phase trip by the earth fault relay is preferable at a single line short-circuit fault.
FIG. 9 is a logic diagram of an example of a trip permitting logic 306 by the short-circuit distance relay shown in FIG. 8. This logic prevents a trip by the short-circuit relay at a single earth fault case. In FIG. 9, the numeral 321 represents a set of under-voltage relays (UV-A, UV-B and UV-C) for phases A, B and C, respectively. The numeral 322 represents a set of two-phase “AND” logics which are turned on when under-voltage relays of at least two phases operate. The numeral 323 represents an “OR” gate logic which is turned on when at least one of the three two-phase “AND” logics operate.
In a typical single line fault, the reduction of the voltages in the healthy phases are small, so that two or more under-voltage relays would not be activated. Therefore, trip can be prevented even when the short-circuit relay has mal-operated in a single line earth fault case, by adding this two-phase “AND” logic condition to the trip permitting logic.
FIG. 10 shows an alternative example of a trip permitting logic shown in FIG. 9. In FIG. 10, reduction in the voltages between each pair of the lines A-B, B-C and C-A are detected, while the voltage reduction in A, B and C-phases are detected by the under-voltage relay 321 in the case shown in FIG. 9. In FIG. 10, the outputs of the three under-voltage relays 324 are input to an “OR” gate logic 323. In case of a single-line earth fault, a trip by the short-circuit distance relay is prevented, because the reduction in voltages between the lines are small.
The conventional device described above can prevent a trip in case of a mal-operation of a short-circuit relay at a one-line earth fault. However, such a device can lead to a three-phase trip by the short-circuit distance relay in case of an internal single-phase fault, if an evolving fault occurred where faults have occurred at an adjacent zone in the forward direction and at an adjacent zone in the reverse direction in a different phase simultaneously or with a time difference.
That is because the condition of phase voltage reduction in two or more phases is fulfilled when single line earth faults have occurred in the forward and the reverse adjacent ends.
In such a case, when a reverse fault has occurred and a permissive trip signal is sent out to the far end of the protected zone of the electric power transmission line, it cannot be decided whether it is a simple two-phase fault by the far end. Therefore, the far end trips in three phases.
Suppose, for example, single-line earth faults have occurred at near ends in different phases in the two lines each in a parallel electric power transmission line, earth faults in A-phase in a first line and in B-phase in a second line have occurred, then both of the two lines are tripped in three phases. That will cause a route cut, which will result in a serious damage in the power transmission system.
In a case where a time difference between an internal fault and a reverse fault of the far end exists, phase defecting state may exist when the reverse fault occurs at the far end. In such a case, so called over-reach phenomenon may occur where the earth fault relay takes the faults located further from the protected zone as a cause of its trip.
Accordingly, it is an object of the present invention to provide an improved directional comparison relay system which can select only the fault phase in the present line and trip the only one phase.
It is another object of the present invention to provide an improved directional comparison relay system which can prevent mal-operation due to over-reaching by making the operation time delayed so that the transient operation may be ignored, when a trip signal already exists or when a trip has already occurred.